In wireless cellular systems represented by W-CDMA (Wideband-Code Division Multiple Access) and LTE (Long Term Evolution) standardized by 3GPP (3rd Generation Partnership Project) or WiMAX (Worldwide Interoperability for Microwave Access) standardized by IEEE (Institute of Electrical and Electronic Engineers), mobile data traffic is accommodated by a large coverage outdoor cell (e.g. macro cell) that is managed by a base station installed outdoors. However, in recent years the amount of mobile data traffic generated indoors has been increasing. To deal with this, installation of indoor base stations for managing indoor cells (e.g., femtocells) having a relatively small coverage have been underway. It is therefore thought that in wireless cellular systems, the traffic environment in which conventionally deployed outdoor cells and newly added indoor cells coexist will grow in the future.
In the wireless cellular system, each base station manages one or more cells while a mobile station establishes radio connection with a cell under which the mobile station resides (exactly speaking, makes a connect with a base station that manages the cell), whereby the mobile station can perform communication with a superior network via the cell. When the mobile station moves from a cell that is connected to a neighboring cell, a process of switching one cell to another, called handover (Handover: HO) is performed. Herein, the cell to which the mobile station is connected is called the source cell, whereas the cell to which the mobile station will be switched is called the target cell.
In the above LTE, in order to perform handover the source base station managing the source cell instructs each mobile station that is connected to transmit a measurement report when a specific event has occurred. An example of the specific event is a case where the radio quality of a cell (which will be referred to hereinbelow as the neighboring cell) adjacent to the source cell becomes better than the radio quality of the source cell. To determine the radio quality of a cell, the received power of the downlink reference signal (RSRP: Reference Signal Received Power), or the received quality of the downlink reference signal (RSRQ: Reference Signal Received Quality), measured by the mobile station, may be used.
The measurement report generated at the mobile station includes the measurement results on radio quality measured by each of the source and neighboring cells. Upon receiving a measurement report from a mobile station, the source base station determines a target cell based on the measurement report. Then, a handover process including signaling for exchanging necessary control information between the target base station managing the target cell and the mobile station is carried out. In most cases, the target cell is selected from the neighboring cells.
Next, description will be made on one example of reporting conditions specified in the 3GPP technical specifications (Non-patent Document 1) in LTE to trigger transmission of a measurement report.
In this technical specification, as one of transmission triggers of a measurement report, Event A3 (Neighbor becomes offset better than serving) is specified. In Event A3 the reporting condition of a measurement report is defined by the following expression (1).Mn+Ofn+Ocn−Hys>Mp+Ofp+Ocp+Off  (1)
Ex. (1) indicates that the radio quality of the neighbor cell (left side) is better than the radio quality of the source cell (right side). Mn in Ex. (1) is the radio quality of the neighboring cell measured by the mobile station. Mp is the radio quality of the source cell. Ofn is the frequency band offset (offsetFreq) of the neighbor cell. Ofp is the frequency band offset of the source cell. Ocn is the cell specific offset of the neighboring cell. Ocp is the cell specific offset of the source cell. Hys is the hysteresis parameter for the reporting condition of Event A3, to be added to the radio quality. Off is the offset parameter to be used for the reporting condition of Event A3, to be added to the radio quality.
Here, the above Ocn and Ocp are configured for each cell pair of the source cell and its neighboring cell. Accordingly, when there exist a plurality of neighboring cells is present, the source base station sets Ocn and Ocp for each neighboring cell. Further, the source base station manages the cell individual offset (CIO: Cell Individual Offset) calculated from Ocn and Ocp by use of the following expression (2), for each cell pair. Since the greater CIO, Ocn becomes a value relatively greater than Ocp, handover from the source cell to the neighbor cell is promoted.CIO=Ocn−Ocp  (2)
Upon determining the reporting condition shown by the above Ex. (1), the source base station notifies the determined reporting condition for each of mobile stations that are connected thereto, to the corresponding mobile station. The reporting condition notified to each mobile station includes the type of the reporting condition (the event type) and parameter values to be used.
The mobile station transmits a measurement report to the source base station when the measurement results of the radio quality of the source cell and the neighbor cell meet the reporting condition of the above Ex. (1) and when the state of the reporting condition that has been satisfied has continued for a predetermined period of time TTT (Time to Trigger) or longer. TTT is also called guard time.
When receiving a measurement report from the mobile station, the source base station determines the target cell for handover based on the measurement report and makes a request for handover (Handover Request) to the target base station managing that cell. When requesting handover, the source base station transmits mobile station information for identifying the mobile station to be handed off, to the target base station. The mobile station information is called UE context in LTE and is information that is used between the base stations that manage the source cell and target cell to identify the mobile station at the time of handover.
Examples of the mobile station information include AS-Config described in Non-patent Document 1 and UE history information described in Non-patent Document 2. AS-Config contains the ID (identification data) of the mobile station, system setting information on the source cell or the like. UE history information is information relating to communication history of the mobile station and contains the ID of, staying time in, the cell to which the mobile station was last connected or the like.
Incidentally, since the above-described handover is a follow-up process that starts the necessary procedures after detecting deterioration of radio quality, there is a risk that the process cannot catch up if an abrupt change of radio quality takes place. If handover fails, the mobile station continues communication with the source cell with the degraded radio quality. As a result, if the radio quality of the source cell degrades to a level lower than the communication permissible quality before the mobile station connects to another cell, a period in which communication is lost (temporary disruption) occurs in the mobile station.
In order to reduce such handover failures, there have been known methods of dynamically adjusting handover parameters such as the aforementioned CIO, TTT and the like.
In LTE, as handover failures, Too Late Handover, Too Early Handover, Handover to Wrong Cell, and Ping-Pong Handover are defined. Too Late Handover is a handover failure that occurs when the time of a handover to the target cell is too late. Too Early Handover is a handover failure that occurs when the time of a handover to the target cell is too early. Handover to Wrong Cell is a handover failure that takes place when, after connection to the target cell that has been determined to be fine in radio quality, the radio quality of the target cell degrades shortly. Ping-Pong Handover is a handover failure in which many handovers are repeated between the source cell and an identical neighbor cell in a short period of time.
As described above, the mobile station transmits a measurement report when the state of the reporting condition of, for example Ex. (1) that has been satisfied has continued for the predetermined guard time TTT or longer. That is, if TTT is set at a large value, the start time of a handover is delayed so that handover becomes unlikely to occur.
A method of reducing handover failures (Too Late Handover, Too Early Handover) by adjusting handover parameters (TTT, CIO, etc.) based on the aforementioned measurement report from the mobile station has been disclosed in Patent Document 1, for example. In Patent Document 1, in reducing Too Late Handover, the CIO value is made large to thereby make the handover time earlier. Further, when the handover failure cannot be resolved if CIO is made greater up to the predetermined upper limit, TTT is changed to a smaller value to make the handover start time earlier, to thereby reduce the occurrence of handover failures.
FIG. 1 is a graph schematically showing the reasons why the occurrence of handover failures is reduced by adjusting the handover parameter (CIO). RSRP and RSRQ characteristics shown in FIGS. 1(a) and 1(b) are the same.
FIG. 1(a) shows how a handover failure occurs due to the handover process starting too late. In FIG. 1(a), the radio quality (RSRP) of the source cell lowers so that the reporting condition defined by the above Ex. (1) holds, then the radio quality (RSRQ) of the source cell becomes lower than a predetermined threshold (required quality) before the lapse of TTT, causing a radio link failure (RLF: Radio Link Failure).
In this case, when the CIO value set for the radio quality (RSRP) of the neighbor cell is made larger, it is possible to make earlier the time when the reporting condition defined by Ex. (1) is satisfied as shown in FIG. 1(b), hence preventing occurrence of RLF.
As mentioned above, it is thought that in the wireless cellular system the traffic environment in which conventionally deployed outdoor cells and newly added indoor cells coexist will grow. In such communication environment including both outdoor cells and indoor cells, the radio quality at a mobile station changes sharply if the mobile station moves between outdoors and indoors. As a result, there occur cases where handover failures cannot be reduced by adjusting handover parameters alone as described above.
This problem in the background art will be described with reference to FIG. 2. The following description will be made by giving an example where the source cell is an outdoor cell and the target cell is an indoor cell.
FIG. 2 is a graph schematically showing the mechanism of change of the radio quality (RSRP, RSRQ) of a mobile station moving from an outdoor cell to an indoor cell.
When the mobile station moves from outdoors to indoors, the RSRP of the source cell (outdoor cell) sharply degrades due to penetration loss upon passage of radio waves through walls etc. because the outdoors and the indoors are partitioned by building walls and the like. On the other hand, the RSRQ of the source cell (outdoor cell) also sharply degrades because of increase of interference waves from the target cell (indoor cell) in addition to the aforementioned penetration loss. Here, “sharp degradation” means that radio quality lowers greater than a specified value within a predetermined period, indicating a case where, for example, RSRP or RSRQ is lower than 10 dB or greater than 10 dB within some hundred milliseconds.
Usually, the time required for a handover in LTE (the time from when the reporting condition defined by Ex. (1) becomes satisfied so that the mobile station transmits the measurement report when the mobile station switches its connection from the source cell to the target cell) is as much as some hundreds milliseconds. In 3GPP, the index of the penetration loss is assumed to be 10 dB or 20 dB.
On the other hand, in the background art for adjusting the aforementioned handover parameters, the upper limit of CIO adjustment is set as high as 6 dB. The reason is that if the upper limit of CIO adjustment is set equal to or higher than 6 dB, there occurs a risk that the mobile station will connect with a cell that cannot meet the quality of radio communication (the necessary quality). If the mobile station connects with a cell that does not meet the necessary quality, the mobile station faces difficulty in signaling with the cell so that the uplink loses synchronization, causing a RLF.
That is, when the mobile station moves from an outdoor cell to an indoor cell, even if CIO is changed to the upper limit of adjustment a RLF takes place hence causing a handover failure (Too Late Handover) because the penetration loss is greater than the upper limit adjustment.
RLF judgment in LTE is effected by the mobile station based on the decision number N310 as a parameter for RLF judgment notified from the base station and decision timer T310. When the radio quality of the source becomes equal to or lower than a predetermined threshold (RLF judgment threshold), the mobile station confirms whether or not the state exceeds the set time of decision timer T310. Then, the mobile station counts the number of times the radio quality has become lower than the RLF judgment threshold after the lapse of the set time of judgment time T310, and when the number of times reaches the above decision number N310, occurrence of a RLF is determined.